1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device and more particularly to a nonvolatile semiconductor memory device having an improved passivation film.
2. Description of the Related Art
Deterioration of the data retention characteristic of a memory cell is a serious problem when the reliability of a nonvolatile semiconductor memory device is considered.
In EPROMs or flash EEPROMs, the state in which charges (electrons) are injected into the charge storage section is called a "programmed" state and data is stored in this state, and in contrast, the state in which charges (electrons) are not injected into the charge storage section is called an "erased" state and data is cleared in this state. If the electrons are moved from the charge storing section, the stored data is lost. Therefore, the data retention characteristic is an important factor in the nonvolatile semiconductor memory device since it is required to hold the data over a long period of time.
Deteriorization of the data retention characteristic is largely dependent on the following two factors.
The first factor is the film quality of an oxide film formed to cover the charge storing section, that is, the floating gate. If the film quality of the oxide film is low, the data retention characteristic is deteriorated. This is because the charges stored in the floating gate are discharged out of the floating gate via that portion of the oxide film which is low in film quality by the effect of the self electric field of the floating gate. As a result, the threshold value of the cell transistor is lowered and the stored data is lost.
The second factor is the presence of mobile ions such as Na.sup.+, K.sup.+ and Li.sup.+. The mobile ions are attracted towards the floating gate by the effect of the self electric field of the floating gate and neutralize the stored charges. This also causes the threshold value of the memory cell to be lowered.
When a BPSG film or PSG film is used as an inter-level insulation film, mobile ions in the semiconductor memory device can be gettered or captured by the film so that deterioration of the data retention characteristic can be prevented. Mobile ions entering from the exterior of the semiconductor memory device can be prevented from further moving towards the floating gate by means of a PSG film used as the passivation film.
However, as the size of the semiconductor memory device is further reduced and the integration density thereof is further increased, the semiconductor device manufacturing process must be effected at low temperatures, and as a result, the reliability of the oxide film is lowered.
Further, as the size and thickness of the element are further reduced, the film thickness of the BPSG film or PSG film is reduced and the wiring interval is reduced so that the film thickness of the PSG passivation film formed on the side wall of the wiring layer will be reduced and the gettering effect by the passivation film will become small. Thus deterioration of the data retention characteristic will develop into a serious problem as size of the semiconductor memory device is further reduced and as the integration density thereof is further increased.